I recently watched the “Tidy eval: Programming with dplyr, tidyr, and ggplot2” video. It’s an excellent introduction to the concept of the tidy evaluation, which is the core concept for programming with dplyr and friends. In this video, Hadley showed on the slide the grouped_mean function (12:48). An attempt to implement this functions might be a good exercise in tidy evaluation, and an excellent opportunity to compare this approach with standard evaluation rules provided by the seplyr package.